Fine carbon fibers represented by, for example, a cylindrical tube type, a fish bone type (cup stack type) and a card-shaped (platelet) type are expected to be used in various applications because of their shape and morphology. In particular, a cylindrical tube type fine carbon fiber (carbon nanotube) has attracted the attention as a next-generation conducting material because it is excellent in properties such as strength and electric conductivity in comparison with conventional carbon materials.
A multilayer carbon nanotube (multilayer concentric cylindrical) (non-fish bone) is described in, for example, Japanese publication of examined application No. H03-64606, Japanese publication of examined application No. H03-77288, Japanese Laid-Open publication No. H09-502487 and Japanese Laid-Open publication No. 2004-299986.
A fish bone type carbon fiber [cup stack type carbon fiber] is described in, for example, U.S. Pat. No. 4,855,091, M. Endo, Y. A. Kim et al., {Appl. Phys. Lett., vol. 80 (2002) 1267-}, Japanese Laid-Open publication No. 2003-073928 and Japanese Laid-Open publication No. 2004-360099. This structure is a stacked open-cup shape.
A platelet type carbon nanofiber (card-shaped) is described in, for example, H. Murayama and T. Maeda {Nature, vol. 345 [No. 28] (1990) 791 to 793}, and Japanese Laid-Open publication No. 2004-300631.
Conventionally known processes for manufacturing a fine carbon fiber representatively exemplified by carbon nanotube include an arc discharge, a vapor growth, a laser and a template methods. Among these, vapor growth using catalyst particles attracts attention as an inexpensive synthetic method, but is not established in a large scale. Furthermore, carbon nanotube produced is a less crystalline inhomogeneous fiber, and therefore, graphitization is necessary when high conductivity is required.
For example, Japanese unexamined patent application publication No. H09-502487 (Patent Reference No. 1) has described that a carbon fibril material (cylindrical tube type) prepared as described in Japanese unexamined patent application publication No. H02-503334 or Japanese Laid-Open publication No. S62-500943 as a prior art has a graphite plane gap (d002) of 0.354 nm as determined by XRD (X-ray diffraction) and is insufficiently crystalline and less conductive. Furthermore, it has been described that by treating this fibril material at 2450° C., a graphite plane gap (d002) becomes 0.340 nm and a fibril material with improved crystallinity is obtained.
In carbon nanotube (multilayer concentric cylindrical type), a graphite-net plane is parallel to a fiber axis, along which electrons flow, so that conductivity is satisfactory in a long-axis direction in a single fiber. However, in terms of conductivity between adjacent fibers, jumping effect by n-electron emission (tunnel effect) cannot occur because a side peripheral surface consists of a cylindrical closed graphite-net plane. There is, therefore, a problem that in a composite with a polymer using carbon nanotube as a conductive filler, if contact between fibers is inadequate, sufficient conductivity is not obtained.
Furthermore, since a cylindrical graphite-net plane in carbon nanotube having this structure consists of a SP2-bond carbon cylinder, it is difficult to cleave strong carbon SP-2 bond by an industrial method generally used (ball mill, bead mill, or the like) for further shortening the fiber to give an industrially available fine short carbon fiber without generating structural defects in the fiber surface.
On the other hand, in a fish bone or platelet type (card-shaped) fine fiber, an open end of a graphite-net plane is exposed in a side peripheral surface, so that conductivity between adjacent fibers is improved in comparison with carbon nanotube. However, it has a stack structure where C-axis in the graphite-net plane is oblique or perpendicular to the fiber axis direction, so that conductivity in a fiber long-axis direction in a single fiber is reduced.
In terms of fiber shortening, a fish bone type carbon fiber has a structure of stacked corn-shaped carbon fundamental planes which are oblique to the fiber axis direction as described in Japanese Laid-Open publication No. 2004-241300 and allows for layer peeling between carbon fundamental planes or interlayer slipping, so that fibers can be easily further shortened. However, since conductivity in the fiber axis direction is extremely low as described above, not only a fish bone type carbon fiber but also a further shortened fiber are unsuitable as a conductive material.
A platelet type (card-shaped) also has a basic structure where stacked carbon fundamental plane disks are perpendicular to the fiber axis as in the fish bone type carbon fiber [cup stack type carbon fiber], and thus, can be easily further shortened, but not only a platelet type carbon nanofiber but also a further shortened fiber are unsuitable as a conductive material because of the reason as in the fish bone type carbon fiber.
In addition to the above structures, Japanese Laid-Open publication No. 2006-103996 (Patent Reference No. 2) has disclosed a fiber structure comprising a structural unit containing a nitrogen atom chemically bonded to a carbon atom in a core of a crystal lattice and composed of a temple-bell-shaped multilayer material in which one end is opened while the other end is closed, wherein the closed end of one unit is inserted to the open end of another unit, as well as a manufacturing process therefor. However, this fiber contains a nitrogen atom chemically bonded to a carbon atom in a graphite-net plane, so that structural distortion is introduced in the graphite-net plane, leading the problem of poor crystallinity, that is, lower conductivity.
Applied Physics A 2001 (73) 259-264 (Ren Z. F. et al) (Non-patent Reference No. 1) has also described a fiber structure called “bamboo-structure” similar to that in Patent Reference No. 2 (Japanese Laid-Open publication No. 2006-103996) described above. This structure is synthesized by vapor growth at 750° C. using a silica-supported iron catalyst and a mixed gas of 20 vol % of acetylene/80 vol % of ammonia. Although chemical composition analysis for the fiber structure is not described at all in this reference, nitrogen which are not inert are contained in a raw material in a very high concentration (59 wt %), so that the fiber structure would also contain a nitrogen atom chemically bonded to a carbon atom, leading to structural disturbance. Furthermore, since a ratio of a product weight to a catalyst weight is as significantly low as about 6, the growth of fiber is insufficient, leading to a lower aspect ratio.
Furthermore, Carbon 2003 (41) 2949-2959 (Gadelle P. et al) (Non-patent Reference No. 2) has also described a structure in which graphite-net planes constituting a fiber have corn shape whose open ends are exposed to a fiber side peripheral surface with proper distance. In this reference, 0.2 g of a mixture of a cobalt salt and a magnesium salt coprecipitated by citric acid is activated by H2 and then reacted with a source gas consisting of CO and H2 (H2 concentration: 26 vol %) to give 4.185 g of a product. However, in a fiber structure obtained by this process, a corn-shaped side peripheral surface forms an angle of about 22° with a fiber axis, that is, it is considerably oblique to the fiber axis. Thus, in terms of conductivity in a long-axis direction of a single fiber, there is a problem as described above for the fish bone carbon fiber. Furthermore, since an aspect ratio is lower due to insufficient fiber growth, it is difficult to give conductivity or reinforcement to a composite with a polymer. Furthermore, since a ratio of a product weight to a catalyst weight is as low as 21, the process is inefficient as a manufacturing process and impurities are contained in a large amount, leading to limited number of applications.
There have been fine carbon fibers having various structures as described above and manufacturing processes therefor, but there are few suggestions for a process for further shortening such a fine carbon fiber. Examples are as follows.
(1) Fibers are cut by applying current within a scanning tunnel microscope (STM) (Non-patent Reference No. 3). In this method, fibers are cut one by one, and therefore, the method is not suitable for large-scale production.
(2) Using a mixed acid of an acid and nitric acid, oxidative decomposition and ultrasonic cutting are simultaneously conducted (Non-patent Reference No. 4). This method has a problem that a carbon wall is significantly damaged. This method also has a problem of yield reduction due to oxidation.
(3) Ball mill cutting (Non-patent Reference No. 5). In this method, the side surface of a fiber is considerably damaged and at the same time, the fiber is contaminated by impurities from a ball mill.
(4) Cutting by heating after fluorination (Non-patent Reference No. 6). This method also has a problem in an yield because a fluorinated moiety is eliminated, and an apparatus becomes larger.
(5) Cutting in a solvent using a high-speed rotation disperser (Non-patent Reference No. 7). This method is convenient, but is not suitable for large-scale production.